1. Field of the Invention
The present invention relates to an electronic flash device used for flash photography by a camera.
2. Description of the Prior Art
An automatic light control electronic flash device used recently in flash photography by a camera employs a thyristor or an FET for ON/OFF-controlling a flash discharge tube. It is known that in a device employing a thyristor, even if control is performed to turn off the flash discharge tube, the OFF timing of the flash discharge tube is delayed, resulting in an excessive quantity of emitted flash light. In a device employing an FET, a large FET element is needed to cope with a large current upon emission of light of the flash discharge tube, resulting in an increase in size of the entire device. For these reasons, Japanese Patent Laid-Open No. 64-17033 proposes an automatic light control flash device which uses a relatively small IGBT (Insulated Gate Bipolar Transistor; described in detail in "Nikkei Electronics", May 19, 1986, No. 395, pp. 182-185) that can perform switching of a large current for turning on/off a flash discharge tube. When an IGBT is used, the flash discharge tube is connected to a main capacitor charged with a DC voltage of, e.g., 280 V, and the cathode of the flash discharge tube is connected to the collector of the IGBT. A light-emitting drive voltage of, e.g., 30 V is applied to the gate of the IGBT to turn on the IGBT, and simultaneously the flash discharge tube emits light when it is triggered in a known manner. When supply of the light-emitting drive voltage to the gate of the IGBT is discontinued, the collector-emitter path of the IGBT is disconnected (OFF), and light emission by the flash discharge tube is stopped. Control of flash light emission is performed by switching the IGBT in this manner.
In this case, in order to obtain the DC voltages of 30 V and 280 V from a power supply cell of a DC voltage of, e.g., 6 V, a known DC-DC converter is generally used, and an AC voltage induced in the secondary winding of the oscillation transformer of the DC-DC converter is rectified.
The conventional device using the IGBT has the following problems.
(1) The AC voltage induced in the secondary winding of the oscillation transformer is rectified to obtain the DC voltage of 30 V as the gate drive voltage to the IGBT. In this case, the AC voltage cannot be efficiently obtained in the secondary winding of the transformer by induction (M), and the size of the oscillation transformer is increased by the capacity of the secondary winding.
(2) For example, a square-wave voltage as shown in FIG. 6(a) is supplied to the gate of the IGBT. However, due to the electrostatic capacitance and the like of the gate-emitter path of the IGBT, the ON (connection)/OFF (disconnection) timings of the collector-emitter path of the IGBT with respect to the rise and fall of the square-wave voltage are delayed, as shown in FIG. 6(b), and the quantity of emitted flash light becomes as shown in FIG. 6(c). In this manner, while the quantity of the emitted flash light is controlled by the ON/OFF operation of the IGBT, since the electrostatic capacitances vary depending on individual IGBTs or other conditions, it is difficult to obtain a desired quantity of emitted light with a uniform light-emitting control signal.